This invention concerns a circuit layout or arrangement for generating a supply DC voltage at an output in a dependent relationship to an input DC voltage applied at the input end, wherein in a first voltage interval the input DC voltage, reduced by a constant first value, is provided, in a following second voltage interval the supply DC voltage is maintained at a constant level, and if the input DC voltage exceeds this second voltage interval, the supply DC voltage at the output end, reduced by a second constant value, follows or tracks the input DC voltage. The invention additionally relates to a process used to generate a supply DC voltage for a signal generator unit where the output voltage is derived from a non-constant input DC voltage and provided to the signal generator unit, preferably with a constant net value, and with the signal generator unit transmitting signals to an evaluation circuit by current pulses superimposed onto the output voltage.
Such circuit layouts are used to supply, e.g., sensors complete with a follow-on signal generator unit with a voltage designed such that variations in the input DC voltage do not constitute a risk or hazard for the functionality of the unit to be supplied. Here, it has proven to be advantageous to maintain the voltage difference--by which the supply voltage for the load lies under the input voltage, as shown in FIG. 1,--at a first constant value, up to a first input voltage value; and to maintain this voltage difference, from a certain input voltage value onwards, constant at a second greater value. In the transition range in between, when in normal mode, the supply voltage will remain constant and be independent of the input voltage.
Such a circuit layout is contained in DE 25 33 199 C3. This circuit layout will generate the described course of a supply voltage in a dependent relationship to the input voltage across a complex transistor circuit, the implementation of which is very laborious and involves very considerable costs.
In addition, DE 41 31 170 A describes a device in which a supply voltage is generated by means of a Zener diode (Z-diode) and a comparator, as well as a controllable current source, which supply voltage will change at intervals in a dependent relationship to the input voltage applied. This layout also proves to be too laborious and costly due to its complexity, in particular the requirement for a controllable current source.
Furthermore, the state of the art knows and comprises additional circuit layouts for voltage stabilization by means of a Z-diode (compare Tietze/Schenk: Halbleiterschaltungstechnik (Semiconductor Circuit Technology), 10th edition 1993, page 555 ff.).
The above-mentioned state of the art also comprises processes for generating such a supply DC voltage.
Here, the supply voltage is gained from a non-constant input DC voltage--such as from a battery, for example,--and provided to the signal generator unit. The signal transmission from the signal generator unit to an evaluation circuit is effected by means of current pulses superimposed upon the supply voltage; the supply DC voltage required for the signal generator unit will preferably be maintained at a constant nominal value which ensures safe signal transmission and signal recognition, and which is also required for circuit elements--sensors, for example,--post-connected to the signal generator unit.
A preferred area of application for such processes is the coupling of decentralized sensor systems with a central electronic control system in motor vehicles whereby the externalized sensors and associated signal generator units will no longer be supplied direct from the onboard power supply but indirectly from the central control unit by means of a current interface. Here, the current variations along the energy supply line to the externalized signal generator unit will be evaluated by a central control unit. Due to the ohmic and capacitive constituents of the sensor and signal generator unit, as well as the electric lines, any voltage change in the central control unit will result in a current change interfering with the superimposed current pulses. Thus, signal evaluation is particularly prone to interference from supply voltage variations.